Movable gate type valves have been used to control the flow of liquids for many years. In modern industrial plants, gate valves are employed most often where on-off control as opposed to variable flow control is required. In large diameter high flow rate or high pressure pipelines gate valves provide for a through conduit having an unobstructed bore which allows smoother, less turbulent flow. Further, pipeline tools such as pigs, scrapers and balls pass easily through such valves.
Gate valves often include a top entry access to allow replacement or service without removal of the valve body from the pipeline. Such gate valves with a flat body or round body typically employ a removable bonnet retained to the valve body by a large number of bolts. To ensure a positive seal under variable pressure and temperature conditions, precision machining of the mating surfaces between valve body and bonnet is required. A seal such as an O-ring seal is often employed. The large number of sealing bolts must be equally torqued to ensure positive sealing under varying pressure and temperature conditions. The O-ring seals are highly susceptible to localized failure if consistent sealing pressure is not maintained.
The servicing of such gate valves is a time consuming and often dangerous process. If the valve body is not completely depressurized before removal of the sealing bolts, the bonnet can blow free after part of the sealing bolts are removed. The large number of bolts and the critical nature of the applied sealing torque results in a disassembly and re-assembly process which is time consuming and requires specialized tools. During re-assembly, the torque of each of the many sealing bolts must be equalized which requires the use of specialized tools to measure torque and bolt elongation to provide a reliable leak resistant seal under variable pressure and temperature conditions. The use of O-ring type seals further requires the removal of the many sealing bolts and the use of a crane to remove the bonnet with related gate and gate control mechanisms to service or replace the O-ring.